Players are free to shuffle and cut their own decks as they choose, though it is recommended to shuffle if able to get a better randomization of cards. Once players have finished with their own decks, they have three options they can do with the opponent's deck. The first option is to do nothing, which is a wise choice if you watched your opponent shuffle thoroughly and believe nothing more needs to be done. If this option is chosen, the opponent must place their deck in the Deck Zone and not touch it again until they draw 5 cards to start a game.
The second option is to cut the opponent's deck. Sometimes an opponent may shuffle well but may or may not notice certain cards rising and staying at the top of the deck. If this option is chosen, carefully take the opponent's deck and take small stacks of cards from it, then put them back together in a different order. After that, the opponent must place their deck in the Deck Zone and not touch it again until they draw 5 cards to start a game.
The final option is to shuffle. Not every opponent is good at shuffling and instead cut their deck a lot or pile shuffle in place of shuffling. This is a good option to help your opponent get their deck shuffled properly, but be careful not to bend their cards or destroy their sleeves in the process. After shuffling the opponent's deck, return it to them, and they have the option to cut it or leave it as is. If they cut, you are allowed one final cut before both players are no longer permitted to shuffle or cut it.